Pollution severity-dependent aerosol light scattering enhanced by inorganic species formation in Beijing haze

The dependence of aerosol optical properties on the chemical composition and size of particles in haze in Beijing was studied. We measured the scattering coefficient of dehydrated PM2.5 aerosols (sigma(sp_dry)) and analyzed the chemical composition of PM2.5. We also monitored the size distribution of particles in the range of similar to 10-700 nm to observe the particle growth (PG(size)). Results showed that the concentrations of secondary inorganic aerosols (SIAs) and the mean size of PM2.5 particles (scattering Angstrom exponent decreasing) increased with the deterioration of the air quality and increase in relative humidity (RH) which enhanced mass scattering efficiency and increased PM2.5. Thus, the increase in sigma(sp_dry) was particularly dramatic and highly sensitive to the ambient RH in severe haze stages. When the ratio of SIAs to PM2.5 (Mm(SIAs)) exceeded 0.35 during the polluted environment, the water content PG(size), and sigma(sp_dry) showed distinct increases, indicating that the formation of SIAs enhanced water vapor condensation and particle growth. This finding revealed the existence of a critical value for M-SIAs in terms of describing the correlation of sigma(sp_dry) variation with pollution severity. The estimation of the respective contributions of individual components to sigma(sp_dry) with the IMPROVE formula revealed that ammonium nitrate and ammonium sulfate were the two largest contributors. These results indicate that the rapid formation of SIAs and PG(size) under humid conditions are the key factors contributing to the increased sigma(sp_dry) via enhanced mass scattering efficiency and increased PM(2.5 )in the severe haze observed in this study. (C) 2020 Elsevier B.V. All rights reserved.